(A) Whole cell lysates harvested from MMTV-PyVmT primary mammary tumor cells were assessed by Western analysis using antibodies indicated. Whole spleen lysates harvested from MerTK+/+ and MerTK–/– mice were used as positive and negative controls, respectively, for MerTK expression. Whole spleen lysates harvested from MerTK+/+ mice were used as a positive control for AXL expression. Whole brain lysates harvested from Tyro3+/+ and Tyro3–/– mice were used as positive and negative controls from TYRO3 expression. (B–D) Bone marrow harvested from MerTK+/+ or MerTK–/– donors was delivered by tail vein injection into lethally irradiated 6-week-old female MMTV-PyVmT recipients. (B) Average tumor volume ± SEM measured in live mice by MRI at 15.8, 17.8, and 19.8 weeks of age. (C) Representative transverse MRI slices of age-matched MMTV-PyVmT recipients of MerTK+/+ or MerTK–/– bone marrow in the lower abdomen/pelvic region. The arrows indicate the location of the spine, while the tumor (T) margins are identified by the red dotted line. (D) Total tumor weight measured at 21 weeks of age (time of necropsy). Horizontal bars represent average total tumor weight ± SEM (n = 14). The P value was calculated using Student’s t test. *P < 0.05. (E) Mammary fibroblasts harvested from MerTK+/+ and MerTK–/– mice were cotransplanted with MMTV-PyVmT tumor cells into the mammary fat pads of MerTK+/+ mice, and tumor latency was measured.